Childhood obesity is a leading public health concern because overweight youth suffer from co-morbidities such as insulin resistance, type 2 diabetes mellitus (T2DM), metabolic syndrome and cardiovascular disease, conditions once considered unique to adults. Although the causes of childhood obesity are not clear, decreased physical activity (PA) is a major factor. In adults, it is well established that regular PA (>150 min/wk) or 30 min/day of PA for 5-7 days/wk is associated with significant reductions in adiposity and prevention of T2DM. However, the potential benefits of this level of PA alone (without calorie restriction) on abdominal obesity, ectopic fat and risk factors for T2DM are unclear in overweight girls. Therefore, we will examine the effects of regular exercise without calorie restriction in overweight girls on: 1) in vivo insulin sensitivity and secretion and risk of T2DM using the clamp technique;2) total and abdominal adiposity using whole body magnetic resonance imaging;3) ectopic fat in the liver and skeletal muscle using 1H-magnetic resonance spectroscopy. More specifically, we will compare the effects of different exercise modalities (aerobic exercise versus resistance exercise) on these health markers, and explore the potential mechanisms by which regular exercise mediates improvements in health risks associated with T2DM. We will recruit 42 sedentary overweight black and white girls, and randomly assign them to one of three 3-month intervention groups: aerobic training (180 min per week), resistance training (180 min per week), or no-exercise control. We believe that the results of this study will be of significance for the development of evidence-based public health and clinical guidelines in our efforts to combat the current epidemic of childhood obesity. PUBLIC HEALTH RELEVANCE: We will examine the effect of a 3-month exercise training (aerobic exercise versus resistance exercise) without dieting on obesity reduction, and improving fitness, body composition and risk of type 2 diabetes in overweight black and white girls.